1. Field of Application
The present invention relates to a rotary electric machine in which a circulating current flows from an armature winding through brushes into field windings of the machine.
2. Description of Related Art
Types of rotary electric machine are known which have an armature formed with four or more poles, with an armature winding lap-wound on the poles, and a field winding connected in series with the armature winding via brushes. With such a rotary electric machine, if differences arise between the levels an electromagnetic force produced between the poles of the armature, a circulating current flows from the armature winding, through the brushes to the field winding. An example of the electrical circuit of such a rotary electric machine is shown in FIG. 2, which will be assumed to operate as a DC motor. This has a field winding formed of four field coils 6a, 6b, 6c, 6d that are connected successively in series, with a pair of positive-side brushes 3a, 3b (i.e., brushes that are at a more positive potential than an opposing pair of brushes, referred to as the negative-side brushes, due to the direction of flow of current into/out of the armature winding) being respectively connected to the connection point 11 of the field coils 6b, 6c and to the connection point 12 of the field coils 6a, 6d, and a pair of negative-side brushes 3c, 3d that are connected to ground potential, i.e., are connected to the low-potential side of a power supply such as a vehicle battery which applies a positive DC supply voltage via a motor lead 7 to the connection point 9 of the field coils 6a, 6b and to the connection point 10 of the field coils 6c, 6d. 
With this configuration, if the potential of the positive-side brush 3a becomes higher than that of the positive-side brush 3b, then a circulating current will flow through the four field coils 6a, 6b, 6c, 6d in the directions indicated by the arrow lines. In this case, the circulating currents that flow in the field coils 6a and 6d will flow in the same direction as the currents supplied from the battery that respectively flow through the field coils 6a, 6d. Conversely, the circulating currents that flow in the field coils 6a and 6d will flow in the opposite direction to the currents supplied from the battery that respectively flow through these field coils.
Hence, the magnetic field strength of the field magnet poles corresponding to the field coils 6a, 6d will become higher than that of the field magnet poles corresponding to the field coils 6b, 6c, i.e., an unbalance will arise between the strengths of the magnetic fields produced by each of the field magnet poles corresponding to the field coils 6a, 6d and the strengths of the magnetic fields produced by each of the field magnet poles corresponding to the field coils 6b, 6c. 
In an attempt to overcome this problem, it has been proposed to incorporate an additional conductor, referred to as a balance winding or balancer, that is connected between the positive-side brushes 3a and 3b for reducing the level of circulating current that flows to the field windings, for example as described in Japanese patent publication No. 08-009578. This proposed balance winding is formed in a semicircular shape, and is disposed along the inner periphery of the yoke, with the opposing ends of the balance winding being electrically connected to and fixedly attached to respective ones of a pair of connection terminals that are coupled to respective connecting leads of the positive-side brushes.
However with such a configuration, it is important to reliably retain the balance winding in a fixed condition. If this is not done, then in the case of a rotary electric machine that is subjected to high levels of external applied vibration (such as a vehicle starter motor), vibration may cause the balance winding to come into contact with the inner surface of the yoke, which can result in short-circuits, or the vibration may cause an open-circuit in the balance winding. Hence in the case of such a type of rotary electric machine that is subjected to severe vibration, it has been necessary to provide a dedicated electrical insulation member for the balance winding, i.e., with the balance winding being embedded within that electrical insulation member. This will result in increased manufacturing cost.
Furthermore with an arrangement in which the balance winding is disposed along the inner circumference of the yoke, the requirements for dimensional accuracy of the balance winding become substantially stringent, which results in further increases in manufacturing cost.